switch dashboard queries to use new bulk DB session

[spatel033]

This change routes dashboard queries to the new 'bulk' database via the session_bulk. We are starting with the dashboards as the first real-world test of the replica connection. This is a low-risk change that can be easily rolled back if users see any issues. This PR will be deployed after initial connectivity test: https://github.com/alphagov/notifications-api/pull/4624. More details on this card.